Common- Don't worry. Depending on how long you have been on your birth control, it is common to have your cycle start early. Continue taking your pills daily and track your cycle, if it becomes more irregular, consult your physician.

Could be sugarpills. Usually the last week of the pills are sugar pills and your menses shows up when the hormone withdraw starts. This could be perfectly normal. Other causes include stress, missed meds. Go see your doc if that is not your case.